1. Field of the Invention
The present invention relates to a method for generating a wobble signal in a CD-R drive and particularly to a method for generating a wobble signal in a CD-R drive in which the influence of the displacement of a main axis of an objective lens of an optical pickup is eliminated.
2. Related Art
A conventional method for generating a wobble signal in a CD-R drive will be described below with reference to FIGS. 3 through 5. As shown in FIG. 3, wobble pits WP displaced left and right from the center of a track are dug in a surface of a CD-R disk so that disk rotation speed control and data write timing control are carried out on the basis of information which is obtained when a three-spot type optical pickup emits a main laser beam MB and sub laser beams SB in the left and right of the main laser beam MB and the main laser beam MB passes through a wobble pit WP.
FIG. 4(a) shows a light-receiving system of an optical pickup in a CD-R drive. The reference numeral 1 designates an objective lens; 2, a polarizing beam splitter; 3, a condensing lens; and 4, a photosensor. As shown in FIG. 4(b), a main-beam photodiode 5 is disposed in the center of the photosensor 4. As shown in FIG. 5, there is extracted a wobble signal which is a push-pull signal of an output from an inner light-receiving portion C1, C4 (upper in the drawing) of the main-beam photodiode 5 and an output from an outer light-receiving portion C2, C3 (lower in the drawing) of the main-beam photodiode 5. EQU wobble signal=(C1+C4)-(C2+C3)
When the objective lens 1 is shifted to the inner or outer side of the disk by an objective lens actuator, however, the main axis of the objective lens 1 is displaced left or right (upper or lower in the drawing) from the center position of the main-beam photodiode 5 shown in FIG. 5(b). Accordingly, the balance between the output from the inner light-receiving portion C1, C4 and the output from the outer light-receiving portion C2, C3 is lost as shown in FIG. 5(a) or 5(c). As a result, a DC offset occurs in the push-pull signal so that it may be difficult to extract the wobble signal.
There arises a technical problem to be solved so that the wobble signal can be extracted securely regardless of the relative positional deviations of the objective lens and the photosensor to thereby improve stability.